1. Field of the Invention
The present invention relates to a camera that is equipped with a vibration compensation device, and in particular, relates to a drive unit for a vibration compensation device in which the amount that a vibration compensation lens is driven is adjusted in accordance with a focal length of a lens and a distance to a subject.
2. Description of the Related Art
A vibration compensation device which compensates for image blur caused by roughly sine-wave shaped vibrations in a camera during photography is disclosed in Japanese Patent Publication HEI 5-40291/1993. A camera is disclosed that is equipped with a conventional vibration compensation device wherein the angular velocity and predicted angular velocity of the vibrations are derived by linear approximation from vibration variation-quantity data output by a vibration detection unit. However, there is no mention of specific vibration periods or of angular velocities or other numerical values.
A conventional vibration compensation device compensates for vibrations in a vibration compensation operation in which an optical axis of a photographic optical system is shifted to suppress the detected vibrations. For example, a vibration compensation lens, which is part of the photographic optical system, is shifted to change the optical axis. If a motor is used as the actuator to drive the compensating lens, the speed of the motor is reduced by gears, etc., and the rotation of the gears are converted to linear movement to drive the vibration compensation lens.
The vibration compensation operation is performed approximately 20 times during one vibration period. Angular velocities and predicted angular velocities of the vibrations are derived by linear approximation from vibration variation-quantity data detected by a pair of vibration detection units. The vibration compensation device compensates for the detected vibrations based on the detected vibration variation-quantities.
Typically, the period of a vibration in the camera, resulting from external forces such as hands, is on the order of 0.1 seconds. Conventional vibration compensation devices can perform 20 vibration compensation operations, approximately one every 5 msec in the vibration period. In order to perform this vibration compensation control, an actuator is needed which has a start up time that is within 5 msec. Practically speaking, small motors having such characteristics do not exist, making it impossible to drive a conventional vibration compensation device using a small motor.
Moreover, using a small motor, in cases where, for example, the movement time constant of the compensating optical system in the vibration compensation device is half the vibration period, the velocity of the compensating lens will not catch up with the angular velocity of the movement of the image, and the vibrational angular velocity decreases before the vibration compensation lens has reached maximum velocity. This may cause the vibration compensation lens to reverse direction, thereby further increasing the blur in the image.
Known vibration compensation devices start operation by reading the position of the vibration compensation lens with a vibration detection unit, such as an interrupter, and thereafter perform a centering action by moving the vibration compensation lens to roughly a center position in a shift range. Next vibration compensation control is started and the vibration compensation lens is driven so as to cancel out the vibrations in response to the output of the vibration detection unit. The vibration compensation control is performed from immediately prior to an exposure process, in which the shutter opens and exposes the film, to just after the exposure process is terminated. The vibration compensation lens is then driven to a prescribed reset position.
However, a problem exists in that a coefficient, which determines how much to change the optical axis relative to the output value from the vibration detection circuit, (termed a "correct compensation coefficient") is a constant value. As long as a subject which is being photographed is at a constant distance, the use of a correct compensation coefficient with a constant value is adequate. When, as is the case normally, the distance to the subject is not a constant value, accurate vibration compensation cannot be performed when the correct compensation coefficient is a constant. In a photographic optical system which uses multiple focal points, or a zoom lens, the correct compensation coefficient must vary with changes in the photographic focal length as well as with changes in the distance to the subject. Also, in the output of the vibration detection unit there are various gain variations, so that the output from the vibration detection unit is not constant but rather various, relative to a specified vibration. The value of the correct compensation coefficient should also be influenced by a gain variation pattern of the vibration detection circuit.
Conventionally, when the correct compensation coefficient is determined, an optical-system compensation quantity for the vibration compensation device is calculated from the correct compensation coefficient and the output of the vibration detection unit. Thereafter, the motor, or other actuator, is driven by a drive quantity that is proportional to the compensation quantity, and the optical axis is changed to compensate for vibrations. However, as set forth above, the vibration compensation device, or, alternatively, the motor or other actuator, has a response delay, or, to be more precise, a wait period from driving the actuator until the vibration compensation device obtains a targeted compensation position or speed. Alternatively, there is a wait time from the detection of the output from the vibration detection circuit until the motor or other actuator is driven. Also, there are fluctuations in a power supply (typically, batteries) that drives the vibration compensation device, characteristic variations in the vibration compensation device, and, differences in the ease with which each mechanism moves relative to a specified driving direction and speed. Such variations and fluctuations can be caused by changes over time or even temperature. Because of these problems, vibration compensation control errors can become large, and precise vibration compensation cannot be effected.
Further, conventionally, in a centering or reset action, a response delay is experienced by the vibration compensation system. Specifically, a wait is experienced from when the actuator driven until a targeted position is obtained. As above, there are fluctuations in the power supply and characteristic variation in operation due to, for example, changes over time or the temperature. As a result, precise vibration compensation control cannot be effected, and in some cases, unpleasant noises are produced due to the operating noise of the motor or actuator operating at uneven speeds. This makes it impossible to move the vibration compensation lens to the center or reset position.
The characteristics of the vibration compensation device markedly deteriorate over time, and can even result in the vibration compensation device becoming completely inoperable. In such cases, precise vibration compensation control cannot be effected, and blurring of the image can be increased. In severe cases, a position detection unit, for detecting the position of the vibration compensation lens, breaks, and the position of the vibration compensation lens can no longer be detected. In this case the vibration compensation lens can be driven beyond the shift region and the vibration compensation device can be damaged.